The present invention is directed to a combined vacuum baffle and valve for a diffusion pump.
FIG. 1 shows a typical prior art diffusion pump. It includes vapor jets 10, only a stage of which is indicated, which may, for example, use oil or mercury vapor diffusing jets to produce the diffusion pumping action. The oil vapor is then condensed on the cold wall 11 of the pump chamber; cooling coils 12 are illustrated. The diffusion pump 13 is connected by flanges 14 to a vacuum chamber 16.
It has been found that diffusion pumps of the present type can be utilized to provide a high vacuum to pressures below 10.sup.-7 torr. This is without the use of cold traps. However, to prevent the transport of the diffusion pump oil or mercury fluid vapors toward the high vacuum chamber 16 it has been found advantageous to use a simple cooled mechanical baffle 17. This prevents the transport of vapor into the vacuum chamber which is known as back-streaming. The center plate 17 provides an optical baffle such that an oil molecule must make a minimum of three bounces from surfaces to pass through the baffle. The surfaces are cooled giving a high probability that the oil molecule will stick to a surface.
Since a diffusion pump removes gas molecules from a vacuum chamber by sweeping down the molecules that come in the path of the oil vapor stream from the vapor jets, minimum impedance should be provided to the gas molecules to minimize the reduction in the effective pumping speed of the diffusion pump. Thus the conductance of baffle 17 should be a maximized.
Since a vacuum often provides a favorable environment for an experiment, frequent access is required to the equipment inside the chamber to make changes; such as, for example, removing finished and loading new components or for general servicing. Before the chamber can be exposed to atmospheric pressure the diffusion pump power must be turned off and the working fluid allowed to cool. During evacuation the diffusion pump does not start to pump until the fluid has reached its operating temperature. Time can be saved by the use of an isolating valve 18 inserted between the vacuum chamber 16 and the diffusion pump 13. Valves of the swing, butterfly, and gate type are among those that have been used.
As illustrated in FIG. 1, for a diffusion pump of speed S.sub.DP the effective pumping speed is reduced by the conductance of the components between the pump, or the vapor jets 10, and the vacuum chamber 16. For a system as illustrated in FIG. 1 with a baffle 17, a valve 18 and a pumping tube 11 all having respective conductances, the effective pumping speed is reduced by these intervening components.